{http://www.bell-labs.com/history/unix/images/kd14.jpeg Dennis Ritchie and Ken Thompson}
'''Dennis Ritchie (standing) and Ken Thompson begin porting UNIX to the PDP-11 via two Teletype 33 terminals.'''

I think it's impossible to provide an exact definition of [Unix].

In summary, Unix is a family of operating systems, comprised of various kernels and suites of basic utilities, and encompassing a philosophy of programming and using computers that has resulted in the Unices of today.

The first versions were implemented at [BellLabs] for a [PDP-7] in 1969.  Today, the trademark ''Unix'' is owned by X/Open, and is licensed to anyone whose system can pass a set of compliance tests.

[Unix] is sometimes called '''*nix''' because at one time AT&T were pretty fascist about protecting the trademark, which they then owned.

For more details, see this (http://cm.bell-labs.com/cm/cs/who/dmr/hist.html paper) or the (http://www.bell-labs.com/history/unix/ corporate glossy) at BellLabs.